这个Haskell定义是什么意思？

Question

Eq a => [a] -> [(Int,a)]

我不知道Eq的含义或含义，所以我不知道整个定义的含义是什么？

Answer 1

That's a constraint about the type a used in the definition [a] -> [(Int,a)] : it says that it must define an instance for the Eq typeclass. Basically, types with an Eq instance give support for equality and inequality operators, (==) and (/=) .

More information about typeclasses here, at learnyouahaskell.com . In nuce, typeclasses are somewhat similar to OOP's interfaces.

Answer 2

这意味着[a]中的元素必须是Eq类型类，这意味着它们必须是可测试的相等。

Answer 3

The items of type a must be of Eq typeclass, as said before. The Eq typeclass defines types which can be compared for equality with other types, with th '==' operator/function

This means that your function takes as its only parameter a list of 'a' items

[a] is a list of types a

and returns a list of (Int, a) pairs, ie a list of 2-tuples where the first item in the tuple is an Int and the second is of same type as the list passed to the function.

(not native english speaker here, sorry :) )